This application is based upon and claims priority of Japanese Patent Application No. 2002-061255, filed on Mar. 7, 2002, the contents being incorporated herein by reference.
(1) Field of the Invention
This invention relates to a semiconductor device and a method for fabricating such a semiconductor device and, more particularly, to a semiconductor device of metal oxide semiconductor (MOS) structure including an insulating film, such as a gate insulator, of two or more different thicknesses and a method for fabricating such a semiconductor device.
(2) Description of the Related Art
Conventionally, silicon oxide films have widely been used as insulating films, such as gate insulators and tunnel insulators, in logic circuits, random access memories (RAMs), erasable programmable read only memories (EPROMs), liquid crystal displays (LCDs), and the like of MOS structure. As these devices become minuter, these insulating films have become thinner. As a result, problems, such as an increase in gate leakage current caused by a direct tunnel current, have arisen. Therefore, in recent years the method of increasing the physical thickness of insulating films by the use of insulating films (high-dielectric-constant films) the dielectric constants of which are higher than that of the conventional silicon oxide films has been proposed.
By the way, voltage applied to elements, such as MOS transistors, is not constant in semiconductor devices in which an LSI circuit is formed. That is to say, some areas operate at high voltage, and others operate at low voltage. Therefore, to decrease a gate leakage current and ensure reliability, MOS transistors in areas where high voltage is applied are thick film transistors in which a thick gate insulator is formed. On the other hand, to improve operation speed and performance, MOS transistors in areas where low voltage is applied are thin film transistors in which a thin gate insulator is formed.
To form a gate insulator of different thicknesses in one semiconductor device, usually the following procedure will be adopted. A silicon substrate is thermally oxidized first to form a thick silicon oxide film. Then a protection film is formed only on an area where a thick film transistor is to be formed, and the silicon oxide film on an area where a thin film transistor is to be formed is removed. After the protection film is removed, the silicon substrate is thermally oxidized again to form a thin silicon oxide film. As a result, a gate insulator of different thicknesses will be formed on the silicon substrate.
To make a gate insulator in a thin film transistor a high-dielectric-constant film, the following procedure, for example, will be adopted. A silicon substrate is thermally oxidized first to form a thick silicon oxide film. Then a protection film is formed on an area where a thick film transistor is to be formed, and the silicon oxide film on an area where a thin film transistor is to be formed is removed. After the protection film is removed, a high-dielectric-constant film is formed by a chemical vapor deposition (CVD) method on the exposed silicon substrate and the silicon oxide film formed by the thermal oxidation. Therefore, a gate insulator in the thin film transistor is of single layer structure of the high-dielectric-constant film and the gate insulator in the thick film transistor is of multilayer structure of the silicon oxide film, being a lower-layer insulating film, and the high-dielectric-constant film formed on it.
However, if a high-dielectric-constant film is formed in a thin film transistor and a multilayer of a silicon oxide film and the high-dielectric-constant film is formed in a thick film transistor, the silicon oxide film formed as a lower-layer insulating film in the thick film transistor may be affected, especially by the influence of chlorine included in gas used as a material for forming the high-dielectric-constant film, resulting in a deterioration in its characteristics as an insulating film.
The present invention was made under the background circumstances as described above. An object of the present invention is to provide a semiconductor device in which the insulation characteristics of an insulating film including a lower-layer insulating film and a high-dielectric-constant film formed on the lower-layer insulating film are ensured without deteriorating the reliability of the lower-layer insulating film, and a method for fabricating such a semiconductor device.
In order to achieve the above object, a method for fabricating a semiconductor device of MOS structure including an insulating film of two or more different thicknesses. This method comprises the steps of forming a silicon oxide film on a semiconductor substrate, removing part of the silicon oxide film formed, and forming a high-dielectric-constant film the dielectric constant of which is higher than that of the silicon oxide film on the semiconductor substrate which has got exposed as a result of removing the silicon oxide film and on the silicon oxide film by the use of a metallic compound not including chlorine.
Furthermore, in order to achieve the above object, a semiconductor device of MOS structure including two or more kinds of insulating films of different thicknesses is provided. This semiconductor device includes a high-dielectric-constant film which is formed by the use of a metallic compound not including chlorine on a semiconductor substrate and a silicon oxide film formed on part of the semiconductor substrate and the dielectric constant of which is higher than that of the silicon oxide film.
The above and other objects, features and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.